G.4 Data transport for DTMF events

26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

For sending and receiving DTMF events via telephone events with RTP, the RTP payload format for "DTMF Named Events" of the telephone-event codec(s), IETF RFC 4733 [61], shall be supported by MTSI clients in terminal and MTSI media gateways.

Telephone events shall use the same RTP media stream as for speech, i.e. the same IP address, UDP port, RTP SSRC and RTP clock rate as the Selected (Speech) Codec. Thereby, RTP Sequence Number and RTP Time Stamp shall be synchronized between RTP packets for speech and RTP packets for DTMF events. For example, by setting the initial random values the same and when switching from speech to DTMF, or vice versa, the RTP Sequence Number and RTP Time Stamp shall continue from the value that was used for the other audio media (speech or DTMF event).

The RTP Sequence Number for telephone events shall increment in the same way as for speech, i.e. by 1 for each transmitted RTP packet.

The RTP Time Stamp for telephone events should increment in the same way as for RTP speech packets or with an integer multiple. Example: if the RTP Time Stamp increments with 160 between RTP speech packets (Codec with RTP clock rate 8000 and 20 ms frame duration) then the RTP Time Stamp increment during DTMF events and when switching between speech and DTMF events should be 160 or an integer multiple of 160. The RTP Time Stamp should not increment with a smaller interval for DTMF events than for speech frames. The RTP Time Stamp shall use the same RTP clock rate as for the speech codec that is transmitted in the same RTP stream immediately before the start of the DTMF event(s).

NOTE 1: A DTMF event may be transmitted in several RTP packets even if the DTMF event has a shorter duration time than what is expressible by the duration field. In this case all RTP packets containing the same DTMF event within the same segment shall have the same RTP Time Stamp value according to IETF RFC 4733 [61].

Speech packets shall not be transmitted such that the resulting decoded speech would overlap in time with the audible representation of DTMF events, when DTMF events are transmitted in the same RTP media stream.

NOTE 2: Most DTMF events are not translated into DTMF tone-pairs, because they are used by the network or an application server (e.g. a voice mail box). For such cases, it would not matter, if DTMF events are overlapping with speech packets, but the non-overlap restriction is kept here for backward compatibility reasons. If an MTSI client receives speech and DTMF event packets, overlapping during a possible playout time, then the MTSI client optimally ignores the overlapping speech packets and plays out only the DTMF events correctly as DTMF tone-pairs, embedded into sufficient silence.

NOTE 3: If the RTP stream carrying the telephone events is paused on RTP level [156], the need to send telephone events may cause the MTSI client in terminal to leave the paused state.

Annex H (informative):
Network Preference Management Object Device Description Framework

This Device Description Framework (DDF) is the standardized minimal set. A vendor can define its own DDF for the complete device. This DDF can include more features than this minimal standardized version. This MO is included in the zip-archive "3GPP MTSI MOs.zip" attached to the present document.

Annex I (informative):
QoE Reporting Management Object Device Description Framework

Annex J (informative):
Media Adaptation Management Object Device Description Framework

Annex K (informative):
Computation of b=AS for AMR and AMR-WB